Recently, utilization of non-azeotropic refrigerant mixtures has been introduced in order to improve the coefficient of performance (COP) of heat pumps. In cases where a non-azeotropic refrigerant mixture is used in a heat pump, a counter-flow type evaporator has usually been employed so as to make the best use of the characteristics of the non-azeotropic mixture. Also, as the non-azeotropic mixture, a mixture containing a relatively larger molar fraction of a low boiling temperature refrigerant than that of a high boiling temperature refrigerant has been used.
However, in certain cases, such as a heat pump system employing, for example, a centrifugal-compressor, it is preferable to use a refrigerant having a certain degree of specific volume and therefore, it has been desired to use a refrigerant mixture containing a high boiling temperature refrigerant as a primary component. However, if a non-azeotropic mixture containing a larger molar fraction of a high boiling temperature refrigerant is used in a heat pump system, a high speed flow of a refrigerant vapor having a large specific volume is induced in the evaporator causing a pressure loss which leads to a power loss.
Also, use of a non-azeotropic refrigerant mixture naturally causes difference in respect of vaporization of the respective components as a result of which a certain component of the refrigerant mixture may stagnate within the system which may reduce the efficiency of the system. More particularly, in an evaporator, a mixture in liquid state containing a relatively high percentage of a high temperature boiling refrigerant is likely to stay at an outlet portion of the evaporator so that heat transfer in the evaporator is impeded compared to the case where a single refrigerant is used. Or, in a condenser, an uncondensed refrigerant mixture which is rich in respect of a low boiling temperature component is likely to stay at a portion adjacent to a refrigerant outlet, thus reducing heat transfer efficiency.
Accordingly, there has been a demand for a heat pump system wherein a non-azeotropic refrigerant is effectively utilized.